The present invention generally relates to electronic duplexers, and more particularly to a method of operating tunable duplexers.
This invention relates to radio frequency and microwave duplexers used in wireless communications transceivers having two channel frequency allocations.
Wireless communications applications have increased to crowd the available spectrum and drive the need for high isolation between adjacent bands. Portability requirements of mobile communications additionally drive the need to reduce the size of communications equipment. Filter and duplexer products are some of the most inevitable components in the radio with requirements to provide improved performance using smaller sized components. Thus efforts have been made to develop new types of resonators, new coupling structures, and new configurations to address these requirements.
Many radio systems use a duplexer to couple the transmit and receive channels to a common shared antenna. Low insertion loss in the two channel passbands and high isolation between the two channels are usually the most important performance requirements of the duplexer. Filter design theory shows, however, that for a given filter frequency mask, optimization of the insertion loss performance often results in degradation of the isolation performance and visa versa. A trade-off between the two parameters is usually required.
Commercially available radio frequency (RF) duplexers include two fixed bandpass filters sharing a common port (antenna port) through a circulator or a T-junction. Signals applied to the antenna port are coupled to a receiver port through the receive bandpass filter, and signals applied to a transmitter port will reach the antenna port through a transmit filter. The receive port and transmitter port are isolated from each other due to the presence of the filters and the circulator, or T-junction. Fixed duplexers are commonly used in point-to-point and point-to-multipoint radios where two-way communication enables voice, video and data traffic within the RF frequency range. Fixed duplexers need to be wide band so that a reasonable number of duplexers can cover the desired frequency plan.
Tunable duplexers could be used to replace fixed duplexers in receivers. A single tunable duplexer could replace several fixed duplexers covering adjacent frequencies. Duplexers that include tunable or switchable filters have been described in U.S. Pat. Nos. 6,307,448; 6,288,620; 6,111,482; 6,085,071; and 5,963,856.
It would be desirable to operate a tunable duplexer in a manner that improves isolation between the transmit and receive channels.
This invention provides a duplexer including a first tunable bandpass filter, a second tunable bandpass filter and means for coupling the first bandpass filter and the second bandpass filter to an antenna. The duplexer is operated by tuning the first tunable bandpass filter to provide a passband corresponding to an assigned transmit frequency, and tuning the second tunable bandpass filter to provide a passband offset from an assigned receive frequency, when the duplexer is operated in a transmit mode. When the duplexer is operated in a receive mode, the first tunable bandpass filter is tuned to provide a passband offset from an assigned transmit frequency and the second tunable bandpass filter is tuned to provide a passband corresponding to the assigned receive frequency.
By using this technique, the isolation between transmit and receive portions of a communications device is improved. The invention also permits the use of filters having a larger passband while maintaining sufficient isolation.